And along with the thrill of a fossil first comes another possible breakthrough. The 47-million-year-old turtle remains offer clues to how a prehistoric lake became one of the world’s richest fossil troves.

"Just finding these couples is completely unique worldwide," lead study author Walter Joyce said. "There are no other vertebrate fossils to be found like this."

The turtle pairs were discovered in Messel Pit, a tropical lake turned Lagerstätten—paleontologist speak for a "really, really, really, spectacular place for fossils," according to Columbia University's Mark Norell.

The prehistoric lake somehow killed scores of animals, then preserved the bodies in volcanic sediment. From those sediments—long since turned to oil shale—nine suspected mating pairs of the Allaeochelys crassesculpta species have been recovered in the last 30 years.

By formally analyzing the fossil pairs for the first time, the study authors were able to determine once and for all that each couple was male-female, in part because males are about 20 percent smaller than females and have longer tails. Perhaps most significantly, the team identified two couples with tails positioned for mating.

"Obviously not," said Norell, who wasn't involved in the research. "Turtles have been around a long time, and they [mate] pretty much now like they did then."

But the study does tell us something about how Messel Pit killed so many ancient animals.

For years paleontologists have hotly debated two hypotheses. One says that sudden, occasional upwellings of carbon dioxide poisoned the lakes. Another says toxic bacteria at the surface were to blame.

Among the evidence for the surface-poison hypothesis are fossils of land animals found near the former lake.

Study leader Joyce, of Germany's University of Tübingen, believes those deaths can be attributed simply to natural causes. "Animals die all the time," he said. "I don't think you need to create a whole theory to support the deaths of a few isolated cases in the area."

The mating-turtle fossils, Joyce added, offer the most solid evidence yet for the carbon dioxide hypothesis.

The turtles, he explained, would have initiated sex only in water that was "intact and not poisonous." Then, in Joyce's telling, they would have sunk during the mating process, as many turtles do. After they'd sunk about 30 feet (10 meters), their permeable skin would have taken in the gas, killing the reptiles.

So, case closed?

"This is science," Joyce said with a laugh. "Most of the time you can't technically prove anything ... this is a very, very reasonable guess."